Known plastic molding machines shape hollow cylindrical material extruded by an extruder. The material is pinched and sealed inside a mold set. Air is blown into the mold. After cooling, the mold set is opened. Generally, these machines have the following structure.
1. Traditional inclined movement of the mold set. In this producing process, the material from the extruder is pinched in the mold set positioned below the extruder head; then, air is blown into the material through a blowing needle cylinder to inflate the material to the mold shape; and then, the material is cut by a cutter located between the head and the mold set. Then, the mold set is slantingly moved down, and in the meantime, the inflated material is cooled to keep its shape. After the mold set is stopped at its lower position, it is opened to allow removal of the finished product. Then, the mold set is moved up again to the head to repeat the process. This kind of structure has the disadvantages and inconveniences listed below.
A. The material has high viscosity and easily softens due to heat. The material often fuses together inside the mold because of the extremely small hollow space between the upper and the lower layer of the material when it is pinched in the mold set and before the blowing needle cylinder is inserted between the layers to inflate the material. Therefore, it often turns out defective products, and such machines are only suitable for products having a comparatively large hollow space in the mold, not for products having an extremely small hollow space.
B. The material located between the mold set and the head is very soft. Hence, the cut edge cannot be straight and clean when cut off. Also, the cut material can rise upward on the mold, and the risen part has to be dealt with as waste.
C. Its production speed is slow, with many defective products and much waste material.
2. Two mold sets alternately move back and forth. While the front mold set receives, pinches the material and moves backward, the rear mold set opens to discharge the finished product and moves forward in a continuous process. This structure has the drawbacks listed below.
A. The material is cut by a cutter after it is pinched in one of the two mold sets. Hence, the problems of waste and correcting the cut edge still remain.
B. It also uses the blowing needle cylinder. It is impossible to produce products having an extremely small hollow space.
C. The process of pinching material in the molds, cutting and blowing the material are not simply done.
D. The structure of alternative operation of the two mold sets is complicated and high in malfunction.
3. Two mold sets are arranged at the right and the left. When the left set retreats after pinching and cutting the material, the right set advances to pinch and cut the material in an alternating continuous process. This structure is quite similar to that of No. 2. Only the moving direction of the mold sets is altered. Hence, it has the same shortcomings as No. 2 does.
As can be understood from the above description, known plastic molding machines are restricted to produce products having a limited height of the hollow space to avoid waste material being fused together before inflation. A height under 0.2mm has never been found in this kind of product. They have the following common traditional drawbacks.
1. The whole structure is not suitable for vertical movement operation.
2. Cutting must be done before moving the mold set for repetitive action due to the non-vertical movement operation.
3. Hollow molding depends on blowing air in.
4. They are not suitable for making products having a flat hollow space.
5. Frequent collecting of waste material is involved.
6. Lengthening the material is impossible.
7. The side surfaces of the product cannot be glossy and smooth.
8. Every edge of the product is alway round, impossible to be made right-angled.
9. Varied layer thickness of the products cannot be effected and controlled in the producing process.